﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;

namespace Pay.Data {
	/// <summary>
	/// Represents the extender for Expression[Func[T, bool]] type.
	/// This is part of the solution which solves
	/// the expression parameter problem when going to Entity Framework by using
	/// Apworks specifications. For more information about this solution please
	/// refer to http://blogs.msdn.com/b/meek/archive/2008/05/02/linq-to-entities-combining-predicates.aspx.
	/// </summary>
	public static class ExpressionFuncExtender {
		#region Private Methods
		private static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge) {
			// build parameter map (from parameters of second to parameters of first)
			var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

			// replace parameters in the second lambda expression with parameters from the first
			var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

			// apply composition of lambda expression bodies to parameters from the first expression 
			return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
		}
		#endregion

		#region Public Methods
		/// <summary>
		/// Combines two given expressions by using the AND semantics.
		/// </summary>
		/// <typeparam name="T">The type of the object.</typeparam>
		/// <param name="first">The first part of the expression.</param>
		/// <param name="second">The second part of the expression.</param>
		/// <returns>The combined expression.</returns>
		public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) {
			return first.Compose(second, Expression.And);
		}
		/// <summary>
		/// Combines two given expressions by using the OR semantics.
		/// </summary>
		/// <typeparam name="T">The type of the object.</typeparam>
		/// <param name="first">The first part of the expression.</param>
		/// <param name="second">The second part of the expression.</param>
		/// <returns>The combined expression.</returns>
		public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) {
			return first.Compose(second, Expression.Or);
		}
		#endregion
	}
}